Rack dependencies preventing thin start - ruby

Question

.rvm/gems/ruby-1.9.3-head/gems/bundler-1.0.21/lib/bundler/runtime.rb:31:in `block in setup': You have already activated rack 1.4.0,
but your Gemfile requires rack 1.2.5. Using bundle exec may solve this. (Gem::LoadError)
Is the error that I get when I try to run thin start , bundle exec thin, rails s on any of my apps. I have tried running system level and app level bundle updates/ installs, removing gemfile.lock and updating again. Sudo level installs. Nothing.
Any advice?
This is my gemfile:
source 'http://rubygems.org'
gem 'rails', '3.0.10'
gem 'rake', '0.8.7'
gem 'jquery-rails', '0.2.6'
gem 'pg', :require => 'pg'
gem 'devise'
gem 'nifty-generators'
gem 'mocha'
gem 'thin'
gem 'omniauth'
require 'oauth2'
gem 'fb_graph'
gem 'nokogiri'
# Bundle edge Rails instead:
# gem 'rails', :git => 'git://github.com/rails/rails.git'
gem 'gravatar_image_tag', '1.0.0.pre2'
gem 'will_paginate', '3.0.pre2'
group :development do
gem 'rspec-rails', '2.5.0'
gem 'faker', '0.9.5'
end
group :test do
gem 'rspec', '2.5.0'
gem 'webrat', '0.7.1'
gem 'spork', '0.9.0.rc5'
gem 'factory_girl_rails', '1.0'
end
# Use unicorn as the web server
# gem 'unicorn'
# Deploy with Capistrano
# gem 'capistrano'
# To use debugger (ruby-debug for Ruby 1.8.7+, ruby-debug19 for Ruby 1.9.2+)
# gem 'ruby-debug'
# gem 'ruby-debug19', :require => 'ruby-debug'
# Bundle the extra gems:
# gem 'bj'
# gem 'nokogiri'
# gem 'aws-s3', :require => 'aws/s3'
# Bundle gems for the local environment. Make sure to
# put test-only gems in this group so their generators
# and rake tasks are available in development mode:
# group :development, :test do
# gem 'webrat'
# end

Answer 1

You can try adding an .rvmrc in your project root with this content:
rvm ruby-1.9.3-head#your-project-name
After that, restart your console session. RVM will ask for permission to use the file. After that, your project will run under it's own gemspace. Run bundle again, and it will install only the required gems.